Coal is an organic material that is burned to produce heat for power generation and for industrial and domestic applications. It has inclusions of mineral matter and may contain undesirable elements such as sulfur and mercury. Coal combustion produces large amounts of ash and fugitive dusts that need to be handled properly. Therefore, run-of-the mine coal is cleaned of the mineral matter before utilization, which also helps increase combustion efficiencies and thereby reduces CO2 emissions. In general, coarse coal (50×0.15 mm) can be cleaned efficiently by exploiting the specific gravity differences between the coal and mineral matter, while fine coal (approximately 0.15 mm and smaller) is cleaned by froth flotation.
In flotation, air bubbles are dispersed in water in which fine coal and mineral matter are suspended. Hydrophobic coal particles are selectively collected by a rising stream of air bubbles and form a froth phase on the surface of the aqueous phase, leaving the hydrophilic mineral matter behind. Higher-rank coal particles are usually hydrophobic and, therefore, can be attracted to air bubbles that are also hydrophobic via a mechanism known as hydrophobic interaction. The clean coal product reporting to the froth phase is substantially free of mineral matter but contains a large amount of water. Wet coal is difficult to handle and incurs high shipping costs and lower combustion efficiencies. Therefore, the clean coal product is dewatered using various devices such as cyclones, thickeners, filters, centrifuges, and/or thermal dryers. In general, the cost of dewatering increases with decreasing particle size and can become prohibitive with ultrafine particles, e.g., finer than 44 μm. In such cases, coal producers are forced to discard them. Large amounts of fine coal have been discarded to numerous impoundments worldwide, creating environmental concerns.
Many investigators explored alternative methods of cleaning fine coal, of which selective agglomeration received much attention. In this process, which is also referred to as oil agglomeration or spherical agglomeration, oil is added to an aqueous suspension while being agitated. Under conditions of high-shear agitation, the oil breaks up into small droplets, collide with coal particles, spread on the surface, form pendular bridges between different coal particles, and produce agglomerates. Nicol, et al. (U.S. Pat. No. 4,209,301) found, that adding oil in the form of unstable oil-in-water emulsions can produce agglomerates without intense agitation. The agglomerates formed by these processes are usually large enough to be separated from the mineral matter dispersed in water by simple screening. Further, selective agglomeration gives lower-moisture products and higher coal recoveries than froth flotation. On the other hand, it suffers from high dosages of oil.
The amounts of oil used in the selective agglomeration process are typically in the range of 5 to 30% by weight of feed coal (S. C. Tsai, in Fundamentals of Coal Beneficiation and Utilization, Elsevier, 1982, p. 335). At low dosages, agglomerates have void spaces in between the particles constituting agglomerates that are filled-up with water, in which fine mineral matter, e.g., clay, is dispersed, which in turn makes it difficult to obtain low moisture- and low-ash products. Attempts were made to overcome this problem by using sufficiently large amounts of oil so that the void spaces are filled-up with oil and thereby minimize the entrapment of fine mineral matter. Capes et al. (Powder Technology, vol. 40, 1984, pp. 43-52) found indeed that the moisture contents were in excess of 50% by weight when the amount of oil used was less than 5%. By increasing the oil dosage to 35%, the moisture contents were substantially reduced to the range of 17-18%.
Keller et al, (Colloids and Surfaces, vol. 22, 1987, pp. 37-50) increased the dosages of oil to 55-56% by volume to fill up the void spaces more completely, which practically eliminated the entrapment problem and produced super-clean coal containing less than 1-2% ash. However, the moisture contents remained high. Keller (Canadian Patent No. 1,198,704) obtained 40% moisture products using fluorinated hydrocarbons as agglomerants. Depending on the types of coal tested, approximately 7-30% of the moisture was due to the water adhering onto the surface of coal, while the rest was due to the massive water globules trapped in the agglomerates (Keller et al., Coal Preparation, vol. 8, 1990, pp.1-17).
Smith, et al. (U.S. Pat. No. 4,244,699) and Keller (U.S. Pat. No. 4,248,698; Canadian Patent No. 1,198,704) used fluorinated hydrocarbon oils with low boiling points (40-159° F.) so that the spent agglomerants can be readily recovered and be recycled. These reagents are known to have undesirable effect on the atmospheric ozone layer. Therefore, Keller (U.S. Pat. No. 4,484,928) and Keller, et al, (U.S. Pat. No. 4,770,766) disclosed methods of using short chain hydrocarbons, e.g., 2-methyl butane, pentane, and heptanes as agglomerants. These reagents also have relatively low boiling points, allowing them to be recycled.
Being able to recycle an agglomerant would be a significant step toward eliminating the barrier to commercialization of the selective agglomeration process. Another way to achieve this goal would be to substantially reduce the amount of the oils used. Capes (in Challenges in Mineral Processing, ed. by K. V. S. Sastry and M. C. Fuerstenau, Society of Mining Engineers, Inc., 1989, pp. 237-251) developed the low-oil agglomeration process, in which the smaller agglomerates (<1 mm) foimed at low dosages of oil (0.5-5%) are separated from mineral matter by flotation rather than by screening. Similarly, Wheelock et al., (U.S. Pat. No. 6,632,258) developed a method of selectively agglomerating coal using microscopic gas bubbles to limit the oil consumption to 0.3-3% by weight of coal.
Chang et al. (U.S. Pat. No. 4,613,429) disclosed a method of cleaning fine coal of mineral matter by selective transport of particles across the water/liquid carbon dioxide interface. The liquid CO2 can be recycled. A report shows that the clean coal products obtained using this liquid carbon dioxide (LICADO) process contained 5-15% moisture after filtration (Cooper et al., Proceedings of the 25th Intersociety Energy Conversion Engineering Conference, 1990, August 12-17, 1990, pp. 137-142).
Yoon et al. (U.S. Pat. No, 5,459,786) disclosed a method of dewatering fine coal using recyclable non-polar liquids. The dewatering is achieved by allowing the liquids to displace surface moisture. Yoon reports that the process of dewatering by displacement (DBD) is capable of achieving the same or better level of moisture reduction than the nal drying at substantially lower energy costs, but does not show the removal of mineral matter from coal.